Cathodic protection is widely used to protect steel by galvanizing this steel with metallic zinc, whose sacrificial corrosion keeps the electrochemical potential of the steel sufficiently negative that the steel does not corrode at all. Cathodic protection of steel has not previously been possible using paint instead of an applied metallic coating, or external sacrificial anodes or applied electric current.
Although a great number of different corrosion-inhibiting paints are known, such inhibiting paints only slow down the onset of corrosion and do not completely and absolutely prevent of corrosion. The present invention, by conferring cathodic protection to the base metal to which it is applied, can entirely prevent the onset of corrosion, and can even stop corrosion that has already started before the paint was applied. The present paint therefore can be applied even over rust and still be effective in preventing corrosion through active cathodic protection.
There are several known paint compositions which disclose the use of metal pigments in combination with pigments, usually carbon or graphite, added specifically to increase the conductivity of the resulting paint. Hardenfelt, in U.S. Pat. No. 4,081,423 teaches a spreading or coating composition including a metallic powder and powdered graphite. In Column 1, lines 27-35 Hardenfelt teaches the use of the metals particularly suited to his invention as being copper, bronze, iron, tin and zinc. Hardenfelt also teaches that the noble metals silver, gold, or platinum may also be used in his invention. All of the metal powders taught by Hardenfelt, except zinc, are relatively electrochemically inert. Zinc is the most electrochemically active of the metals taught by Hardenfelt, but even zinc has an electrochemical potential of only -1.0 volts when measured versus a calomel electrode in seawater. It has now been found that a substantial portion of the electrochemical activity of the zinc is lost by the incorporation of this zinc into a paint, even an electrically conducting paint, and as a consequence zinc cannot provide complete cathodic protection when it is thus incorporated into the paint because its electrochemical activity is effectively reduced by such incorporation. None of the metals taught by Hardenfelt will, when used in a paint, confer complete cathodic protection to steel in seawater because of the loss in the electrochemical activity of the metal pigment material which occurs when the pigment is incorporated into the paint.
Corboy, et al., in U.S. Pat. No. 4,490,282, teaches in the abstract a paint composition comprising a hardenable fluid binder, metal particles selected from the group of nickel particles, aluminum particles and mixtures thereof, carbon particles, 2, 4-pentadione and a titanate coupling agent. Corboy does not anywhere teach the use of metal pigments of extremely high electrochemical activity.
Pollart, et al., in U.S. Pat. No. 5,002,826, teaches, in the abstract, coating compositions comprising finely divided carbon, a microwave reactive metal, powdered inert solid and a binder. Pollard, et al., cite preferred metals as being aluminum, iron, nickel, copper, tin and silver, all of which are of relatively low electrochemical activity. Even aluminum, which has an electrochemical potential of -0.98 volts versus a saturated calomel reference electrode in seawater, will not provide complete cathodic protection because of the loss in its electrochemical activity which occurs when it is incorporated into paint. None of the metals taught by Pollart, et al. will, when used in a paint, confer complete cathodic protection to steel in seawater because of the loss in the electrochemical activity in the metal pigment material which occurs when the pigment is incorporated into the paint. Nowhere do Pollard, et al., suggest the use of metallic pigments having an exceptionally high electrochemical activity to achieve active cathodic protection or the combined use of an active metal pigment in combination with a corrosion inhibiting agent to produce an electrochemically active paint of extended active lifetime.
Fouts, Jr. et al., in U.S. Pat. No. 4,545,926, teaches, in the abstract, a polymer composition comprising a polymeric material having dispersed therein conductive particles composed of a highly conductive material and a particulate filler. Fouts, Jr., et al. teach specifically the use of nickel, tungsten, molybdenum, silver, gold, platinum, iron, aluminum, copper, tantalum, zinc, cobalt, chromium, lead, titanium and tin, and cites particularly the preferred use of nickel, tungsten, and molybdenum. None of the metals taught by Fouts, et al., are of high electrochemical activity and none would produce complete cathodic protection of iron or steel in seawater.
Japanese Patent Specification No. 57-085994 teaches, in the abstract, an electroconductive resin composition comprising a thermoplastic resin, polyurethane, carbon black, sulphur and a metal or metal sulphide powder. The metal powders include Ni, Fe, Zn, Cr, Cu, and Al. Nowhere does this specification disclose the incorporation of extremely active metal pigments.